The present invention relates to a semiconductor device for controlling a switching power supply. Particularly it relates to a semiconductor device for controlling a switching power supply, in which brown-out detection can be made without provision of any special input terminal for detecting brown-out (i.e., a voltage drop in an electrical power supply that may cause malfunction of a circuit).
There has been heretofore known a switching power supply control semiconductor device such as a switching power supply control IC having a power supply terminal Vcc, a mute terminal M for suspending a switching operation, an output terminal OUT and a feedback terminal FB, in which a switching element is operated by an output of a voltage detection circuit for detecting an input voltage input to a primary coil of a transformer so that the mute terminal M connected to the switching element is short-circuited to the ground through a resistor to suspend the operation of the main switching element (see JP-A-2006-14465).
There has been also heretofore known, for example, a switching power supply control semiconductor device having an input voltage detection unit for detecting an input voltage, a timer for obtaining an operating time longer than a cycle of an AC voltage by a time constant defined by a capacitor, a charging/discharging unit for charging/discharging the capacitor, and a timer control unit for controlling the charging/discharging unit based on the input voltage detected by the input voltage detection unit, wherein a brown-out detection function is provided so that the value of the input voltage input to a primary coil of a transformer can be detected surely during the operation of the timer while charging/discharging of the capacitor of the timer can be controlled by the timer control unit based on the voltage value detected by the input voltage detection unit (see JP-A-2006-304485).
There has been further heretofore known a DC-DC converter having a brown-out detection function in which abnormal reduction in an input voltage input to a primary coil of a transformer and an output voltage obtained from a secondary coil of the transformer is detected quickly to halt the switching operation and keep the halt state stably even when the input voltage increases temporarily after the switching operation is halted by reduction in the input voltage (see JP-A-2001-258249).
There has been further heretofore known a switching power supply control semiconductor device for turning on/off a switching element connected between a primary-side main winding wire of a transformer and the ground to supply electric power to a load connected to a secondary side of the transformer, including a power supply terminal which is externally connected with a capacitor and to which a power supply voltage is input from a primary-side auxiliary winding wire of the transformer, a starting terminal to which a voltage supplied to the primary side of the transformer is input, a charging starter element which allows a starting current to flow from the starting terminal to the power supply terminal to charge the capacitor, a control circuit which controls so that the starting current flows at startup, and a starter circuit provided with a starting current regulating circuit which keeps the starting current constant (see JP-A-2006-204082).
Although each of the switching power supply control semiconductor devices disclosed in JP-A-2006-14465, JP-A-2006-304485, and JP-A-2001-258249 was designed to detect the input voltage input to the primary coil of the transformer, it was necessary to provide a pin dedicated for brown-out detection and a pin dedicated for starting current inflow and charging, respectively and individually. For this reason, there was a problem that the number of pins in the switching power supply control semiconductor device increased. Because the number of pins in a semiconductor device always needs to be reduced, the increase in the number of pins is not desirable in view of the needs.
In the switching power supply control semiconductor device disclosed in JP-A-2006-204082, a starter circuit is provided but there is no technique concerning reduction in the number of pins in the switching power supply control semiconductor device.
In the switching power supply control semiconductor device disclosed in JP-A-2006-204082, the number of pins can be reduced if brown-out can be detected by a current inflow and charging at terminal VH, but there is a problem as the following. That is, when an AC (alternating current) power source for supplying an input voltage to a primary-side coil of a transformer is used to allow a starting current to flow into the starter circuit at startup, the input voltage from the AC (alternating current) power source is applied directly to the high voltage input terminal VH. It is therefore necessary to provide a current limiting resistor RVH having a high resistance of several KΩ in order to prevent abnormality such as short-circuiting between the high voltage input terminal VH and the ground GND. When the starter circuit is on, the starting current flows into the starter circuit via the current limiting resistor RVH so that a voltage drop occurs in the current limiting resistor RVH. For this reason, the voltage on the high potential side of the current limiting resistor RVH can be hardly detected by the brown-out detection comparator. Accordingly, to avoid this trouble, a pin dedicated for brown-out detection and a pin dedicated for starting current inflow and charging need be provided as an integrated circuit in the same manner as in the switching power supply control semiconductor device disclosed in each of JP-A-2006-14465, JP-A-2006-304485, and JP-A-2001-258249. Consequently, there was a problem that the number of pins increased because such a pin dedicated for brown-out detection and a pin dedicated for starting current inflow and charging were required.
Therefore, in order to solve the aforementioned problems, one of the objectives of the present invention is to provide a semiconductor device for controlling a switching power supply, in which a voltage detection terminal for brown-out detection is used in common as a current inflow terminal for a starter circuit so that brown-out detection is enabled without provision of any special input terminal for brown-out detection.
Further objects and advantages of the invention will be apparent from the following description of the invention.